Galectins are proteins with a characteristic carbohydrate recognition domain (CRD) (Barondes et al., 1994; Leffler et al., 2004). This is a tightly folded β-sandwich of about 130 amino acids (about 15 kDa) with the two defining features 1) a β-galactose binding site and 2) sufficient similarity in a sequence motif of about seven amino acids, most of which (about six residues) make up the β-galactose binding site. However, sites adjacent to the β-galactose site are required for tight binding of natural saccharides and different preferences of these give galectins different fine specificity for natural saccharides.
The recent completion of the human, mouse and rat genome sequences reveal about 15 galectins and galectin-like proteins in one mammalian genome with slight variation between species (Leffler et al., 2004)
Galectin subunits can contain either one or two CRDs within a single peptide chain. The first category, mono-CRDs galectins, can occur as monomers or dimers (two types) in vertebrates. The by far best studied galectins are the dimeric galectin-1, and galectin-3 that is a monomer in solution but may aggregate and become multimeric upon encounter with ligands (Leffler et al., 2004). These were the first discovered galectins and are abundant in many tissues.
There are now over 3500 publications on galectins in PubMed, with most, as mentioned above, about galectins-1 (>900) and -3 (>1600). Strong evidence suggests roles for galectins in e.g. inflammation and cancer, and development recently reviewed in a special issue (Leffler (editor), 2004b).
Galectins are synthesized as cytosolic proteins, without a signal peptide on free ribosomes. Their N-terminus is acetylated, a typical modification of cytosolic proteins, and they reside in the cytosol for a long time (not typical of secreted proteins). From there they can be targeted to the nucleus, specific cytososlic sites, or secreted (induced or constitutively) by a non-classical (non-ER-Golgi) pathway, as yet unknown, but possibly similar to the export of e.g. IL-1 (Leffler et al., 2004). They can also function in all these compartments; for galectin-3, solid evidence published in well respected journals support roles in RNA splicing in the nucleus, inhibition of apoptosis in the cytosol, and a variety of extracellular effects on cell signaling and adhesion (Leffler (editor), 2004b). Galectin-7 and -12 also act in the cytosol by enhancing apoptosis and regulating the cell cycle and differentiation in certain cells (Hsu and Liu in Leffler (editor), 2004b). Most galectins act also extracellularly by cross-linking glycoproteins (e.g. laminin, integrins, and IgE receptors) possibly forming supramolecular ordered arrays (Brewer et al., 2002) and may thereby modulate cell adhesion and induce intracellular signals. Related to this, recent years have seen the emergence of a molecular mechanism of these galectin functions involving a formation of microdomains (lattices) within membranes, (Dam et al., 2008; Garner et al., 2008) which in turn affects intracellular trafficking and cell surface presentation of glycoprotein receptors. (Delacour et al., 2007; Lau et al., 2007; Lau et al. 2008) This has been documented in cell culture, in null mutant mice, (Blois et al., 2007; Gedronneau et al., 2008; Thijssen et al., 2007; Toscano et al., 2007; Saegusa et al., 2009) and animals treated with galectin (Blois et al., 2007; Perone et al., 2009) or galectin inhibitors. (John et al., 2003; Pienta et al., 1995; Glinsky et al., 1996)
Potential Therapeutic Use of Galectin-3 Inhibitors
Galectin-3 has been implicated in diverse phenomena and, hence, inhibitors may have multiple uses. It is easy to perceive this as a lack of specificity or lack of scientific focus. Therefore, the analogy with aspirin and the cyclooxygenases (COX-I and II) is useful. The COXs produce the precursor of a wide variety of prostaglandins and, hence, are involved in a diverse array of biological mechanisms. Their inhibitors, aspirin and other NSAIDs (non-steroid anti-inflammatory drugs), also have broad and diverse effects. Despite this, these inhibitors are very useful medically, and they have several different specific utilities.
So if galectins, like COXs, are part of some basic biological regulatory mechanism (as yet unknown), they are likely to be ‘used by nature’ for different purpose in different contexts. Galectin inhibitors, like NSAIDs, are not expected to wipe out the whole system, but to tilt the balance a bit.
Inhibition of Inflammation
A pro-inflammatory role of galectin-3 is indicated by its induction in cells at inflammatory sites, a variety of effects on immune cells (e.g. oxidative burst in neutrophils and chemotaxis in monocytes), and decrease of the inflammatory response, mainly in neutrophils and macrophages, in null mutant mice (in Leffler (editor), 2004b). Moreover, knock-out mice of Mac-2BP, a galectin-3 ligand, have increased inflammatory responses (Trahey et al., 1999). Importantly, recent studies have identified galectin-3 as a key rate-limiting factor in macrophage M2 differentiation and myofibroblast activation, which influences the development of fibrosis (Mackinnon et al., 2008; Mackinnon et al., 2012).
Inflammation is a protective response of the body to invading organisms and tissue injury. However, if unbalanced, frequently it is also destructive and occurs as part of the pathology in many diseases. Because of this, there is great medical interest in pharmacological modulation of inflammation. A galectin-3 inhibitor is expected to provide an important addition to the arsenal available for this.
Treatment of Fibrosis-Related Conditions
The idea of a possible role of galectin-3 in fibrosis comes from cell and ex vivo studies on macrophage differentiation (Mackinnon et al., 2008), as well as from in vivo studies on macrophage differentiation and myofibroblast activation (Mackinnon et al., 2012). Briefly, the hypothesis is as follows: Galectin-3 has been shown to prolong cell surface residence and thus enhance responsiveness of the TGF-β receptor (Partridge et al., 2004), which in turn regulates alternative macrophage differentiation into M2 macrophages and myofibroblast activation. Hence, as galectin-3 is a good candidate for being an endogenous enhancer of TGF-β signaling and alternative macrophage differentiation and myofibroblast activation, galectin-3 inhibitors may be very useful in treating fibrosis and adverse tissue remodeling.
Treatment of Cancer
A large number of immunohistochemical studies show changed expression of certain galectins in cancer (van den Brule et. al. and Bidon et al. in Leffler (editor), 2004b) and for example galectin-3 is now an established histochemical marker of thyroid cancer. The direct evidence for a role of galectin-3 in cancer comes from mouse models, mainly by Raz et al, but also others (in Leffler (editor), 2004b). In paired tumor cell lines (with decreased or increased expression of galectin-3), the induction of galectin-3 gives more tumors and metastasis and suppression of galectin-3 gives less tumors and metastasis. Galectin-3 has been proposed to enhance tumor growth by being anti-apoptotic, promote angiogenesis, or to promote metastasis by affecting cell adhesion. From the above it is clear that inhibitors of galectin-3 might have valuable anti-cancer effects. Indeed, saccharides claimed but not proven to inhibit galectin-3 have been reported to have anti-cancer effects. In our own study a fragment of galectin-3 containing the CRD inhibited breast cancer in a mouse model by acting as a dominant negative inhibitor (John et al., 2003). More recently, inhibition of galectin-3 with small molecules have been demonstrated to indeed greatly enhance tumor cell sensitivity towards radiation and standard pro-apoptotic drugs in cell assays and ex vivo (Lin et al., 2009), as well as in vivo (Glinsky et al., 2009).
Also galectin-1 is frequently over-expressed in low differentiated cancer cells, and galectin-9 or its relatives galectin-4 and galectin-8 may be induced in specific cancer types (Huflejt and Leffler, 2004; Leffler (editor), 2004b). Galectin-1 induces apoptosis in activated T-cells and has a remarkable immunosuppressive effect on autoimmune disease in vivo (Rabinovich et al; and Pace et al. in Leffler (editor), 2004b). Therefore, the over-expression of these galectins in cancers might help the tumor to defend itself against the T-cell response raised by the host.
Null mutant mice for galectins-1 and -3 have been established many years ago (Poirier, 2002). These are healthy and reproduce apparently normally in animal house conditions. However, recent studies have revealed subtle phenotypes in function of neutrophils and macrophages (as described above) and in bone formation for galectin-3 null mutants, and in nerve and muscle cell regeneration/differentiation for the galectin-1 null mutants (Leffler et al., 2004; Poirier, 2002; Watt in Leffler (editor), 2004b). Recently galectin-7 and galectin-9 null mutant mice have been generated and are also grossly healthy in animal house conditions, but have not yet been analyzed in detail. The differences in site of expression, specificity and other properties make it unlikely that different galectins can replace each other functionally. The observations in the null mutant mice would indicate that galectins are not essential for basic life supporting functions as can be observed in normal animal house conditions. Instead they may be optimizers of normal function and/or essential in stress conditions not found in animal house conditions. The lack of strong effect in null mutant mice may make galectin inhibitors more favorable as drugs. If galectin activity contributes to pathological conditions as suggested above but less to normal conditions, then inhibition of them will have less unwanted side effects.
Treatment of Angiogenesis
Vascular endothelial growth factors (VEGFs) signaling through VEGF receptor-2 (VEGFR-2) is the primary angiogenic pathway. Studies have been published demonstrating that both galectin-1 (Gal-1) and galectin-3 (Gal-3) are important modulators for VEGF/VEGFR-2 signaling pathway. It has also been published that a galectin inhibitor, TDX, is expected have efficacy against pathological angiogenesis. (Chen 2012)
Known Inhibitors
Natural Ligands
Solid phase binding assays and inhibition assays have identified a number of saccharides and glycoconjugates with the ability to bind galectins (reviewed by Leffler, 2001 and Leffler et al., 2004). All galectins bind lactose with a Kd of 0.5-1 mM. The affinity of D-galactose is 50-100 times lower. N-Acetyllactosamine and related disaccharides bind about as well as lactose, but for certain galectins, they can bind either worse or up to 10 times better. The best small saccharide ligands for galectin-3 were those carrying blood group A-determinants attached to lactose or LacNAc-residues and were found to bind up to about 50 times better than lactose. Galectin-1 shows no preference for these saccharides.
Larger saccharides of the polylactosamine type have been proposed as preferred ligands for galectins. In solution, using polylactosamine-carrying glycopeptides, there was evidence for this for galectin-3, but not galectin-1 (Leffler and Barondes, 1986). A modified plant pectin polysaccharide has been reported to bind galectin-3 (Pienta et al., 1995).
The above-described natural saccharides that have been identified as galectin-3 ligands are not suitable for use as active components in pharmaceutical compositions, because they are susceptible to acidic hydrolysis in the stomach and to enzymatic degradation. In addition, natural saccharides are hydrophilic in nature, and are not readily absorbed from the gastrointestinal tract following oral administration.
Galectin Specificity
The studies of galectin specificity using inhibition by small natural saccharides mentioned above indicated that all galectins bound lactose, LacNAc and related disaccharides, but that galectin-3 bound certain longer saccharides much better (Leffler and Barondes, 1986). These longer saccharides were characterized by having an additional sugar residue added to the C-3 position of galactose (in e.g. lactose or LacNAc) that bound an extended binding groove. The shape of this groove varies between galectins, suggesting that the same extensions would not be bound equally by the different galectins.
Synthetic Inhibitors
Saccharides coupled to amino acids with anti-cancer activity were first identified as natural compounds in serum, but subsequently, synthetic analogues have been made (Glinsky et al., 1996). Among them, those with lactose or galactose coupled to the amino acid inhibit galectins, but only with about the same potency as the corresponding underivatized sugar. A chemically modified form of citrus pectin (Platt and Raz, 1992) that inhibits galectin-3 shows anti-tumor activity in vivo (Pienta et al., 1995; Nangia-Makker et al., 2002).
Cluster molecules having up to four lactose moieties showed a strong multivalency effect when binding to galectin-3, but not to galectin-1 and galectin-5 (Vrasidas et al., 2003). Cyclodextrin-based glycoclusters with seven galactose, lactose, or N-acetyllactosamine residues also showed a strong multivalency effect against galectin-3, but less so against galectins-1 and -7 (André et al., 2004). Starburst dendrimers (André et al., 1999) and glycopolymers (Pohl et al., 1999; David et al., 2004), made polyvalent in lactose-residues, have been described as galectin-3 inhibitors with marginally improved potency as compared to lactose. The aforementioned synthetic compounds that have been identified as galectin-3 ligands are not suitable for use as active components in pharmaceutical compositions, because they are hydrophilic in nature and are not readily absorbed from the gastrointestinal tract following oral administration.
Natural oligosaccharides, glycoclusters, glycodendrimers, and glycopolymers described above are too polar and too large to be absorbed and in some cases are large enough to produce immune responses in patients. Furthermore, they are susceptible to acidic hydrolysis in the stomach and to enzymatic hydrolysis. Thus, there is a need for small synthetic molecules
Thiodigalactoside is known to be a synthetic and hydrolytically stable, yet polar inhibitor, approximately as efficient as N-acetyllactosamine (Leffler and Barondes, 1986). N-Acetyllactosamine derivatives carrying aromatic amides or substituted benzyl ethers at C-3′ have been demonstrated to be highly efficient inhibitors of galectin-3, with unprecedented IC50 values as low as 4.8 μM, which is a 20-fold improvement in comparison with the natural N-acetyllactosamine disaccharide (Sörme et al., 2002; Sörme et al., 2003b). These derivatives are less polar overall, due to the presence of the aromatic amido moieties and are thus more suitable as agents for the inhibition of galectins in vivo. Furthermore, C3-triazolyl galactosides have been demonstrated to be as potent inhibitors as the corresponding C3-amides of some galectins. Hence, any properly structured galactose C3-substituent may confer enhanced galectin affinity.
However, the C3-amido- and C3-triazolyl-derivatised compounds are still susceptible to hydrolytic degradation in vivo, due to the presence of a glycosidic bond in the galactose and N-acetyllactosamine saccharide moiety and, although they are potent small molecule inhibitors of galectin-3, even further improved affinity and stability is desirable. Accordingly, inhibitors based on 3,3′-diamido- or 3,3′-ditriazolyl-derivatization of thiodigalactoside have been developed, (Cumpstey et al., 2005b; Cumpstey et al., 2008; Salameh et al., 2010; WO/2005/113569 and US2007185041; WO/2005/113568, U.S. Pat. No. 7,638,623 B2) which lack O-glycosidic hydrolytically and enzymatically labile linkages. These inhibitors also displayed superior affinity for several galectins (down to Kd in the low nM range). Nevertheless, although displaying high affinity for galectins, the 3,3′-derivatized thiodigalactosides still comprise a disadvantage in their multistep synthesis involving double inversion reaction to reach at 3-N-derivatized galactose building blocks. Furthermore, cyclohexane replacement of one galactose ring in thiodigalactoside has been evidenced to mimic the galactose ring and hence to provide galectin-1 and -3 inhibitors with efficiency approaching those of the diamido- and ditriazolyl-thiodigalactoside derivatives (WO/2010/126435). Replacement of a D-galactopyranose unit with a substituted cyclohexane decreases polarity and most likely also metabolic susceptibility, thus improving drug-like properties.
Some earlier described compounds have the following general formulas
as described in WO/2005/113568,and
as described in WO/2005/113569, in which R1 can be a D-galactose, and
as described in WO/2010/126435.
Thus, due to the less than optimal manufacturing processes towards galactose 3-N-derivatization (Z and Y are preferably nitrogen atoms) involving double inversion reactions at a complex protected D-galactopyranose derivative of the compounds of the prior art, there is still a considerable need within the art of inhibitors against galectins, in particular of galectin-1 and galectin-3.